Duplicating stencils



United States Patent 3,151,548 DUPLECATWG STENCILS Henry John Picking, London, England, aseignor to Gestetner Limited, London, England, a British company Filed Apr. 15, 1964, Ser. No. 360,056 Claims priority, application Great Britain Apr. 2?, 1963 14 (Ziairns. (Q1. 101-4282) This invention relates to electrically cut duplicating stencils.

One known type of stencil adapted to be cut electrically comprises a paper backing coated with a highly conductive carbon-containing layer and a stencil sheet of thin porous paper (the stencil tissue used in conventional typing stencils) impregnated with nitrocellulose loaded with carbon to give an electrically conductive layer of lower conductance than the aforesaid highly conductive layer, the stencil sheet being mounted on the same side of the backing as the highly conductive layer. This type of stencil sufiers from the disadvantage that the backing sheet is black and cannot therefore be used in the way in which conventional, mechanically cut stencils are used, to show on the duplicating machine when the stencil has become impregnated with ink and running ofi can begin. A further disadvantage of this type of stencil is the extent to which set oif (transfer of ink from one recently duplicated copy to the back of the next) occurs.

Another known type of electrica ly cut stencil comprises a paper backing having a double layer of a vinyl chloride polymer film strippably bonded thereto, both layers of polymer film being loaded with carbon to render them electrically conductive, that adjacent the backing having (as in the first type of stencil) the higher conductivity. In use, the double film is perforated electrically and then stripped from the backing. This type of stencil requires expensive equipment to cast the very thin selfsupporting vinyl chloride polymer films needed. Moreover the film is liable to tear on stripping and inevitably shrinks after the stripping operation. A further disadvantage is that used stencils of this type have to be stored in such a way as to prevent loss of plasticiser if further shrinkage and tendering are to be avoided.

The present invention provides a novel type of electrically cut duplicating stencil free from the disadvantages inherent in the types of stencil mentioned above.

The duplicating stencil of the present invention comprises a backing sheet having attached thereto a stencil sheet made of paper less than 0.00125 inch thick (which can be wet strengthened) of porosity not greater than 250 (determined by the Gurley-Hill SPS method), and loaded with particles of an electrically conductive material, and coated on the side adjacent the backing sheet with a highly conductive layer capable of being cut electrically and having a conductivity of 4 l0 to 2 l0*' mhos, when determined by the method hereinafter described, and on the opposite side with a relatively less conductive layer also capable of being cut electrically.

The backing sheet can be of the kind ordinarily used on duplicating stencils and will normally be attached to the stencil sheet along one edge provided with perforatic-as or other means for attaching the stencil to the duplicating machine. A scored line near this edge enables the backing sheet to be easily torn away from the stencil sheet when the latter is in position on the duplicating machine. For reasons given above the backing sheet will ordinarily be white or pale-coloured. In place of a conventional backing sheet of the kind mentioned or as a separate sheet between the stencil sheet and the backing, it is possible to use a lithographic master sheet having a hydrophilic surface. If this is done then, during the cutting of the stencil, material is transferred from the cut Patented Get. 6, 1964 "ice areas of the stencil to the surface of the lithographic master in a pattern which is a duplicate of the cut pattern in the stencil sheet itself. Since the transferred material is oleophilic with the stencil sheets of the kind ordinarily used in this invention, it follows that the lithographic master itself having a hydrophilic surface will become coated with an oleophilic image and can be used to produce reproductions of this image by the ordinary lithographic technique. Duplicating stencils in accordance with the present invention modified in this way are as such a feature of the invention. The lithographic master sheet may be of paper or metal or of other conventional kind. I

The paper used in the stencil sheet must be thin and of very low porosity. If it is too thick it cannot be satisfactorily cut electronically. A practical maximum thickness is 0.00125 in., and a thickness of about 0.001 inch is preferred. if it is too porous, there is liable to be some irregular interpenetration of the two conductive layers and the desirable properties of the new stencil will be marred or lost. The maximum porosity is 250 as determined by the Gurley-Hill SPS method. This method is based on a determination of the time in seconds taken for cc. of air to penetrate the paper under standard conditions. It will be noted that a high figure corresponds to a low porosity and vice versa. Thus a paper having a maximum porosity of 250 has a porosity which numerically has a minimum value of 250. The preferred porosity is about 1500. The weight of the paper will ordinarily be governed by the need to satisfy the necessary conditions of thickness and porosity. A paper weighing about 10-12 g./rn. gives good results.

The thin, non-porous paper is preferably wet-strengt ened so as to make the stencil capable of enduring long runs on the duplicator. This can be done with any synthetic resin useful for wetstrengthening paper, e.g. ureaand melamine-formaldehyde resins, but resins such as polyamides, water insolubilisable polyacrylates and acrylate copolymers, polyacrylic esters (e.g. polymethyl methacrylate), polystyrene and its copolymers, and natural and synthetic elastomers are preferred because they ha a no deleterious effect on the electrical properties of the paper. The resins are incorporated in the paper during its manufacture as aqueous dispersion, emulsions (latices) or solutions. If the resin is initially water-soluble, it must be rendered insoluble in situ in the paper. The particularly preferred resin is a polyamide modified with an epihalohydrin, e.g. that sold under the trade name Kymene 660 by the Hercules Powder Company, which is the product obtained by condensing a polyamide with epichlorohydrin so that the secondary amino groups react with the formation of an azidine ring (see United States Patent No. 2,926,154).

The paper is loaded with electrically conductive particles by adding the latter as a paste to the paper-making pulp in the beater. From 5% up to about 30% by weight of the paper of the conductive particles may be added, about 10% being preferred. Particles of carbon black are preferably used but metal powders are also operable, the concentration of particles depending on their particular electrical conductivity.

The highly conductive layer capable of being cut electrically may be a layer of highly conductive particles, e.g. of carbon black, dispersed in a film-forming binder, which is, if necessary, plasticized. Polyvinyl chloride copolymer is preferred but cellulose nitrate, polyvinyl formal, polyvinyl acetate copolymers, polymethyl meth acrylate and its copolymers can, for example, all be used.

The layer of lower electrical conductivity is similar in composition to the highly conductive layer except that its conductivity is reduced by using either less of the conductive particles or particles of lower intrinsic conductivity, or both.

The conductivity of the highly conductive layer is 4X10 to 2 1O- mhos, preferably 4 10 to 1 10 mhos, as determined using two brass electrodesone cm. apart, each 1 cm. square and under a load of 2 kilograms. The conductivities of the paper and the less conductive layer are not critical but are ordinarily much lower than that of the highly conductive layer. Thus the paper has a conductivity, measured in the same Way as that mentioned above but on paper having the highly conductive layer on the reverse side, which is usually 1.6 10* to 2 l0 mhos or even lower, preferably 1 10- to 6X10 mhos. Similarly the layer of lower conductivity has a conductivity, measured in the same way on a complete stencil sheet, i.e. in the presence of both paper and highly conductive layer, which is less than that of the paper and is ordinarily 1.4 10 to 2 10- mhos and is preferably 6x10- to 5 10 mhos. It should be borne in mind that it may sometimes be desirable to'vary the conductance of the upper layer (that of lower conductivity) so as to match the output of the electronic scanning device used to cut the stencil. The figures given however suit the type of machine most commonly in use.

The two conductive layers are conveniently coated on the base paper by dispersing the coating composition in a solvent for the binder, for example acetone when the binder is polyvinyl chloride, and applying the dispersion to the paper by a known coating method, egg. by roller coating or slot coating. An advantage of the new type of stencil is that it can be made using the same coating and other machinery used formaking conventional, mechanically-cut stencils. Expensive equipment is not required as the coated layers do not have to be self-supporting.

The two conductive layers together should not be more than 0.0005 in. thick so that the total thickness of stencil should not exceed 0.00175 in. 1

In their preferred form the new stencils possess the advantage that their use involves little or no set ofi during duplication. This is due to the fact that the new stencils can be made very thin, and thinner, for example, than prior known stencils based on stencil tissue, and

have in addition, a surface that can be described as planographic.

If desired the upper surface of the stencil (the layer of lower conductivity) may be provided with a white or grey top coat to render the characters on the cut stencil more visible. Such a coat may have acomposition similar to those of the conductive layers and be applied in a similar way, except that the carbon is replaced by a white or grey pigment, e.g. titanium dioxide. While these top coats have the advantage mentioned, they are liable to be a source of wastage in manufacture and the preferred stencils therefore do not include them.

The new stencils have a property which is sometimes advantageous,,narnely that they can be cut mechanically, e.g. with a wheel pen or with a file plate and stylus. This makes it possible to cut the new stencils partly electrically and partly mechanically, so that, for example, additions may be made to a sheet being copied.

The following example illustrates the invention.

Example The stencil sheet is tissue weighing 11 g./sq. rn., having (on evaporation of the acetone) a highly conductive layer.

j Wet-strengthened paper less than 0.00125 inch thick, of

7 Parts by weight Polyvinyl chloride copolymer (Breon 425 of British Geon) 100 Carbon black (Vulcan XC 72 of Cabot Carbon) 60 Tri-tolyl phosphate l5 V/hite lead (stabilizer for copolymer) 7 500 Acetone The opposite side of the stencil sheet is then coated with the following composition to give (on evaporation of the acetone) a layer of lower conductivity.

Parts by weight Polyvinyl chloride copolymer (Breon 425) 100 Carbon black (Dixie of Anchor Chemical) 25 Tri-tolyl phosphate 27' White lead stabilizer 8 Acetone 500 The coated stencil sheet thus obtained is then fixed to a conventional stencil backing sheet with the highly con-' than 250 determined by the Gurley-Hill SPS method and.

loaded with particles of an electrically conductive material, a highly conductive layer capable of being cut electrically and having a conductivity of 4 10- to 2 10 Inhos coated on said paper on the side adjacent said backing sheet, and a relatively less conductive layer also capable of being cut electrically coated on said paper on the opposite side to said backing sheet.

'2. A stencil according to claim 1, in which the said paper has a conductivity of 1X10 to 6X10- mhos and the less conductive layer a conductivity of 6 l0 to' 5 X 10- mhos.

3. A stencil according to claim 1, in which the stencil sheet is made of paper about 0.001 inch thick and having a porosity of about 1500 determined by the Gurley-Hill SPS method.

4. A stencil according to claim 1, in which the stencil sheet is made of paper loaded with 5 to 30% by weight of electrically conductive particles.

5. A stencil according to claim 4, in which the paper is loaded with carbon black. 7

6. A stencil according to claim 1, in which the stencil sheet is made of paper which has been wet-strengthened.

7. A stencil according to claim 6, in which the paper has been wet-strengthened with a polyamide modified with an cpihalohydrin; 7

8. A stencil according to claim 1, in which the highly conductive layer and the relatively less conductive layer are layers of a film-forming binder having electrically con ductive particles dispersed therein.

9. A stencil according to claim 8, in which the said layers are layers of polyvinyl chloride having carbon black dismrsed therein.

10. A stencil according to claim 9, in which the combined thickness of the inch.

11. A'stencil according to claim 1 provided with a pale coloured top coat.

12. A stencil according to claim 1 provided with a.

lithographic master having a hydrophilic surface adjacent to the stencil sheet'on the side next to the backing sheet.

13. A duplicating stencil comprising a backing sheet, a

stencil sheet attached to said backing sheet and made of porosity not greater than 250 determined by the Gurley-.

said layers is not more than 0.0005

Hill SPS method, loaded with 5 to 30% by weight of electrically conductive particles and having a conductivity of 1.6 10- to 2X10 rnhos, a highly conductive layer of a film-forming binder having electrically conductive particles dispersed therein and havin a conductivity of 4X10 to 2 l0- mhos coated on said paper on the side adjacent said backing sheet, and a relatively less conductive layer of a film-forming binder having electrically conductive particles dispersed therein and having a conductivity of 1. l 10- to 2x10 mhos coated on said paper on the opposite side to said backing sheet, the C0111- bined thickness of the said layers being less than 0.0005 inch.

14. A duplicating stencil comprising a backing sheet, a stencil attached to said backing sheet and made of paper about 0.001 inch thick, wet-strengthened with an epihalohydrin-modified polyamide, having a porosity of about 1500 determined by the Gurley-Hill SPS method, loaded with 5 to 30% by weight of carbon black particles and having a conductivity of 1 l0- to 6 10- mhos, a highly conductive layer of a polyvinyl chloride binder having carbon black particles dispersed therein and having a conductivity of 4X 10 to 1X10 mhos coated on said paper on the side adjacent said backing sheet, and a relatively less conductive layer of a polyvinyl chloride binder having carbon black particles dispersed therein and having a conductivity of 6 10-' to 5 10 mhos, coated on said paper on the opposite side to said backing sheet, the combined thickness of the said layers being less than 0.0005 inch thick.

References Cited in the file of this patent UNITED STATES PATENTS 2,528,005 Kline Oct. 31, 1950 3,113,511 Dalton Dec. 10, 1963 FOREIGN PATENTS 4 Great Britain of 1879 1,055,558 Germany Apr. 23, 1959 

1. A DUPLICATING STENCIL COMPRISING A BACKING SHEET, A STENCIL SHEET ATTACHED TO SAID BACKING SHEET AND MADE OF PAPER LESS THAN 0.00125 INCH THICK, OF POROSITY NOT GREATER THAN 250 DETERMINED BY THE GURLEY-HILL SPS METHOD AND LOADED WITH PARTICLES OF AN ELECTRICALLY CONDUCTIVE MATERIAL, A HIGHLY CONDUCTIVE LAYER CAPABLE OF BEING CUT ELECTRICALLY AND HAVING A CONDUCTIVITY OF 4X10**-3 TO 2X10**-4 MOHOS COATED ON SAID PAPER ON THE SIDE ADJACENT SAID BACKING SHEET, AND A RELATIVELY LESS CONDUCTIVE LAYER ALSO CAPABLE OF BEING CUT ELECTRICALLY COATED ON SAID PAPER ON THE OPPOSITE SIDE TO SAID BACKING SHEET. 